This invention relates to linear actuators suitable for positioning a payload, such as an antenna element for accurate orientation of a beam of radiation provided by the antenna and, more particularly, to a set of differentially coupled linear actuators connected to an antenna mount wherein a second of the actuators can act in concert or as a backup to the primary actuator.
Antennas used in communication systems, particularly satellite communication systems, must be accurately positioned to insure that a narrow pencil beam is oriented in a desired direction. In a multi-element antenna, such as an antenna employing both a main antenna and a subreflector which illuminates the main antenna with radiation from a feed, adjustment of the orientation of the subreflector itself can refine the beam definition as well as the beam orientation. The general direction of the beam can be established in the construction of the satellite wherein the antenna is given a specific orientation relative to the satellite and, then, upon placing the satellite into orbit, refinements in the position of the satellite serve to direct the antenna accurately in a desired direction. Further adjustment of a beam shape and direction can be accomplished electronically in the case of phased array antenna. However, it may still be advantageous to provide for mechanical adjustment of the antenna position, or orientation of an element of the antenna, particularly if the antenna feed and possibly reflectors of the antenna have been configured to provide a specific configuration of beam.
Mechanical scanners of antennas have been used for many years to provide for scans such as an azimuthal scan. Such scanners are generally large and heavy which would militate against their use in a satellite and, furthermore, may not be suitable to provide the very fine adjustment in orientation of an antenna or an element thereof as is required for a satellite borne antenna. A further requirement for satellite operation is reliability such as may be afforded by a positioning system employing redundant positioning devices. A problem arises in that presently available positioning apparatus does not offer the function of very fine accuracy over a moderate range of scan angle in combination with the features of small size and weight in conjunction with redundancy, especially mechanical redundancy.